Numerous methods are practiced for the metallic plating or coating of substrates, usually for anti-corrosive and/or decorative purposes or to provide a wear resistance or a conductive layer. These methods include the application of metallic coatings to substrates by electro-chemical and chemical deposition, in tanks of aqueous solutions of metal salts, by immersion of substrates into molten baths of the coating metal and by spray and brush application of the coating metal to substrates in the form of a paint.
Although these prior art techniques have enjoyed some commercial success, they are not without their shortcomings. Many of these prior art techniques, for instance, involve the use of hazardous chemicals and/or fumes or result in hydrogen embrittlement. Others are characterized by less than desirable deposition rates or the requirement for complex and costly equipment. Most prior art techniques do not effect any mechanical working or hardening of the substrate surface.
One prior art technique has provided a safe, simple and inexpensive method for the application of a metal coating to a substrate while simultaneously hardening the substrate surface. This method has employed substantially standard shot-peening apparatus to apply a mixture of peening particles and metallic coating powders to the substrate material. In this method, the peening particles drive the metallic coating particles onto the substrate while simultaneously mechanically working (hardening) the substrate surface. This method has proved useful with a broad range of substrates, traditional peening particles such as glass beads and metal shot, and various conventional metal powders made from ingot metallurgy. Illustrative of metallic powders that may be used in this method are aluminum, nickel, silver, gold, tungsten, copper and zinc. The prior art did not employ this method for the application of rapidly solidified (R-S) metal coatings because this method was deemed by some in the art to be inapplicable to the application of R-S metal powders which are made directly from molten metal and are generally characterized as very fine powders with either amorphous or microcrystalline structure. As the mechanism is understood, the energy (work) produced by the peening particles at the substrate surface has been considered by some to be insufficient to create a bond of the R-S metal powder to the substrate surface.
R-S metal powder coatings have been of interest for a considerable period of time because the metal is endowed with a refined microstructure and microchemistry and, consequently, the resulting structure is superior in terms of mechanical properties such as strength, wear and corrosion resistance. Moreover, R-S metals, whether in the amorphous or microcrystalline state, form superior anti-corrosive layers which are long-lasting, both at room and at moderately high temperatures. Prior art methods of applying R-S metal powders to substrates as a coating have included sputtering, ion plating, laser glazing and ion implantation. In spite of the continuing interest in R-S metals and resulting extensive research in this area, none of these methods have proved commercially viable because of such factors as complexity, cost and undesirable results. As an example, laser glazing has not wide acceptance because this process results in the formation of micro-cracks, thereby resulting in a non-uniform surface. Further, the process presents safety problems, requires highly trained personnel, is complex, expensive and time consuming and not adapted to large surface applications.